This invention relates to the field of shaped holes, to the field of laser drilled holes, and particularly relates to the field of laser drilled shaped holes for film cooling of gas turbine hardware.
The efficiency of gas turbine engines can be increased by increasing operating temperatures. Current practice is to operate gas turbines at gas temperatures which can exceed the melting point of the materials used in the combustion and turbine. Melting is avoided by providing air cooling.
In a common cooling scheme known as film cooling, pressurized air is flowed into passages within the component and exits through a plurality of finely drilled holes which communicate between the passage and the surface to be cooled. The most effective cooling is obtained when the cooling air exiting the holes flows smoothly, as a film, over the surface to be cooled. In so doing the cooling air insulates the surface from the hot gas stream. The effectiveness of this type of cooling requires that the air film remain immediately adjacent to the surface to be cooled. In terms of art, the cooling air flow remains xe2x80x9cattachedxe2x80x9d to and does not xe2x80x9cseparatexe2x80x9d from the surface. Effective cooling also requires that the film cover as much of the surface as possible.
The air used to cool the parts reduces engine efficiency and there is a need to minimize the amount of cooling air and maximize its effectiveness. Various hole geometries have been proposed to accomplish these ends. Typically the holes are drilled at an acute angle to the surface to be cooled so that the air exiting the cooling hole has a natural tendency to flow over the surface in the direction of the external air flow. Some proposed cooling schemes have involved shaped holes, e.g., straight sided, elliptical holes, which provide a fan of cooling air rather than a cylindrical column of cooling air.
It has also been proposed to use holes which vary in cross section as they pass through the thickness of the part being cooled. Thus, for example in U.S. Pat. No. 3,527,543 it is proposed to use small conical holes which are divergently tapered over their entire length toward the outer surface. This allows the cooling air to expand in a controlled fashion so that its velocity is reduced and its area increased, thus rendering it more likely to remain attached to the surface being cooled and providing greater surface coverage.
Two general techniques are used for cooling hole production. Electro-discharge machining (EDM) uses an electric arc to erode the workpiece as the current flows through a shaped electrode. As the shaped electrode is advanced into the workpiece, arc discharge occurs and a hole is produced which closely mimics the external shape of the electrode. Such holes are slow and expensive to produce.
U.S. Pat. Nos. 4,197,443 and 4,705,455 deal with the production of shaped cooling holes having a tapered portion and a constant cross section portion using an electro-discharge machining technique. Electrodes are used which produce holes which have an essentially rectangular exit geometry.
The other common hole drilling technique is laser drilling. In this technique a pulsed laser with a focused beam produces a hole by melting the substrate. Laser drilling is substantially cheaper and faster than EDM machining. Techniques have been proposed to produce shaped holes with laser drilling, see for example U.S. Pat. No. 4,992,025, but none have met with commercial success because they increase the time and cost of laser drilling and the complexity of the laser drilling apparatus.
U.S. Pat. Nos. 4,762,464 and 4,808,785 propose the combination of EDM and laser drilling to produce cooling holes.
According to the present invention, shaped cooling holes are produced by a multi-step laser drilling process.
The hole has a straight sided metering portion, with an essentially constant cross section, which originates at the inner surface of the component to be cooled and transitions to a cone shaped portion whose large end is located on the surface to be cooled. The straight sided portion provides metering of the air. The conical portion permits expansion and slowing of the cooling air, known as diffusion, so that upon exit from the conical section the air flow tends to become and remain attached to the surface rather than separate from the surface as might be the case if a straight sided metering hole extended to the outer surface.
The laser drilling is accomplished with a pulsed laser. In a preferred method the laser beam has a relatively large spot size and high total energy when the conical portion is formed. When the conical section has been drilled, the workpiece is moved relative to the beam so that the beam size is smaller. The energy of the beam is also reduced and the metering portion of the hole is drilled.